The invention relates to a method for dissolving an oxide layer containing chromium, iron, nickel, and optionally zinc and radionuclides using an aqueous oxidative decontamination solution containing permanganic acid and a mineral acid, flowing in a circuit (K1), wherein the oxidative decontamination solution is adjusted to a pH ≤2.5, especially for decomposing oxide layers deposited on the interior surfaces of areas or components of a nuclear power plant.
The invention particularly relates to a method for extensive decomposition of the radionuclides in the primary system and the auxiliary system in a nuclear power plant using the available operating medium and the operating system of the power plant itself.
During the generating operation of a PWR (pressurized water reactor) nuclear power plant, with an operating temperature of >180° C. and reducing conditions on the interior surfaces of the systems and components wetted by the medium, oxidic protective layers (Fe0.5Ni1.0Cr1.5O4, NiFe2O4) are formed. In this process, radionuclides are incorporated into the oxide matrix as well. The goal of chemical decontamination methods is to break down this oxide layer in order to remove the incorporated radionuclides. The goal of this procedure is to minimize the radiation exposure of the maintenance staff in case of a maintenance operation insofar as possible, or in the case of dismantling of the nuclear reactor, to allow the components to be returned to a recycling program without problems.
The oxide protective layers are not removable chemically based on their composition and structure. Using a prior oxidative chemical treatment of the oxide structure, these can be broken up and the difficult-to-dissolve oxide matrix converted to readily soluble metal oxides. This breaking up of the oxidized matrix is done by oxidation of the trivalent chromium to hexavalent chromium:Fe0.5Ni1.0Cr1.5O4/NiFe2O4/Fe3O4→Oxidation→CrO42−,FeO,NiO,Fe2O3   Equation (1)
Throughout the world, so-called permanganate preoxidation according to equation (2) has become used as an oxidation treatment, wherein the following three oxidation treatments are available:“NP” oxidation=nitric acid+potassium permanganate (nitric acid,permanganate) (see, for example, EP-B-0 675 973)“AP” oxidation=sodium hydroxide+potassium permanganate (alkaline,permanganate) “HP” oxidation+permanganic acid (see, for example, WO-A-2007/062743),Mn-VII+Cr-III→Mn-IV+Cr-VI  (Equation 2)
The manganese ion is present in permanganate at an oxidation number of 7 and is reduced to an oxidation number of 4 according to equation (2), while at the same time the chromium, present in the trivalent oxidation state, is oxidized up to an oxidation number of. 6. According to equation (2), 2 mol MnO4− are required for the oxidation of 1 mol Cr2O3,
Chemical decontamination of an entire primary system including all activity-carrying auxiliary systems was previously performed only in a few nuclear power plants. In recent years about 50 different decontamination methods were developed worldwide. Of all these methods, the only technologies that became widely used were those based on initial preoxidation with permanganates (MnO4−), e.g., (EP 0 071 336, EP 0 160 831 B1, EP 242 449 B1, EP 0 355 628 B1, EP 0 753 196 B1, EP 1 082 728 B1).
Available chemical decontamination methods are fundamentally performed with the following process sequence at this time:
Step I: preoxidation step
Step II. reduction step
Step III. decontamination step
Step IV: decomposition step
Step V: final cleanup step.
All methods use permanganate (potassium permanganate, permanganic acid) for preoxidation (I.) and oxalic acid for reduction (II.). The methods only differ in the decontamination step (III.). Different chemicals and chemical mixtures are used here.
The decontamination methods to date are based on the previously explained concept. The poorly soluble oxide protective layers are converted in a preoxidation step into more readily soluble oxide compounds and remain on the surface of the system. Therefore no removal of activity from the systems to be decontaminated takes place during the preoxidation. No decrease in the radiation dose rate takes place during this time phase of decontamination with existing methods.
Only after the second process step (II.) of the reduction of the permanganate and the manganese dioxide formed with oxalic acid and in the decontamination step (III.) are the oxides dissolved and the dissolved cations/radionuclides removed and bound to ion exchange resins.
During the preoxidation (I.) in all previously used decontamination technologies, manganese oxyhydrate [MnO(OH)2] and manganese dioxide (MnO2) form, as is clearly shown by equations (3) and (4).2MnO4−+Cr2O3+H2O→2MnO(OH)2+2CrO42−+H2O2MnO(OH)2→2MnO2+2H2O  (Equation 3)(AP/HP-Oxidation)4KMnO4+4HNO3+2Cr2O3+4H2O→4MnO(OH)2+4KNO3+2H2Cr2O7 4MnO(OH)2→4MnO2+4H2O  (Equation 4)(NP-Oxidation)
The manganese dioxide is insoluble and deposits on the inner surface of the components/systems. With increasing manganese oxyhydrate/manganese dioxide deposition, the desired oxidation of the oxidic protective layer is impeded. In addition the converted iron and nickel oxides remain undissolved on the surface, so that the barrier layer on the surface is further thickened.
At the end of the preoxidation step, the following new chemical compounds, introduced or formed in process step (I.), are present:                On the system surface: MnO2, NiO, FeO, Fe2O3, Fe3O4         
In the preoxidation solution: KMnO4, NaOH or HNO3, colloidal MnO(OH)2, CrO42− and Cr2O72−.
Thus at the end of the preoxidation step, all metal oxides including the radionuclides are still present in the system to be decontaminated. Part of the manganese oxyhydrate formed [MnO(OH)2] was carried into system areas without flow passing through and can no longer be carried out/removed in the subsequent process steps.
According to the prior art, no decrease in radioactivity occurs, thus no decontamination, occurs during oxidation of the oxide layer, since practically no cations that could be removed with the aid of the cation exchanger are dissolved out of the oxide layer. Instead the breakdown of the oxide layer is accomplished in a second process step with the aid of oxalic acid, preceded by a reduction step for reducing excess permanganic acid and manganese oxyhydrate. Only after these process steps are cations removed from the cleanup solution (decontamination solution) by ion exchange.